I may be charging headlong up a blind alley (please correct me if I am) but I'm interested in trying to capture differences between the theoretical ratio of Hα to Hβ. Even with a 3nm Hα filter there is a good chance of contamination by NII which, as you know, is extremely close to Hα so if I measure NII, also with a 3nm filter, then I should be able to get a better Hα/Hβ ratio. As I understand it the chances are good that a deviation of that Hα/Hβ ratio will be caused by dust preferentially absorbing the bluer Hβ and it's those subtle dust clouds I'm actually after. My idea of "distinctiveness" as expounded in your book.

I'm very definitely up for correction if any of my assumptions above are wrong or if the number of suitable targets where dust may be revealed in that way are so small as to make the Hβ investment not worthwhile, both in terms of (a lot of) money spent and about three times the imaging time at Hβ compared to Hα to get a similar S/N ratio. If that's the case then I'd rather opt for SII first and defer purchase of an NII filter as, if memory serves, the number of targets where NII shows extra detail is relatively small. Am I making sense?

For someone that hasn't done this stuff before - you really impress the hell out of me with the homework you've done

What you're saying all makes perfect sense - and there I was thinking I was the only one out there that thought H-beta was a good thing

Dust preferentially scatters (rather than absorbs) the shorter (bluer) wavelengths - hence all the reflection nebulosity around the Pleiades. This blue is broadband so you aren't helped much using a narrowband filter to try and get the dust (or the Integrated Flux nebula).

BUT - going back to your original idea of caprturing the differences between H-alpha and H-beta, yep, go for it.

I've mentioned PixInsight in a few of my more recent posts in this thread. I was nearing the end of my trial licence and, frankly, I can't imagine being without it now so I sprung a couple of hundred Euros of my hard earned on a full licence. I'm pleased to say that, despite having to uninstall the trial version, when I installed the commercial version and entered my new licence details in to activate the software my previous customisation was still in place.

I've not researched all the options but I've tried and had some success with the free IRIS software. Despite having to buy a copy of Maxim DL to get the best out of my FLI CCD camera I have tried and totally failed to get on with that software's image processing routines. But PixInsight stands tall in their company and they offer a generous fully functional free 45 day trial. Recommended.

After what seems like forever the weather gods smiled last night, although the smile was slightly cynical as it turned misty at about half past midnight curtailing the session slightly. Anyhow, I managed to capture 18 "lights" of 2 minute exposure each with my 5D2 at ISO 400 and my 85mm lens wide open at f/1.2. An IDAS light pollution filter was used in front of the lens.

I commanded the 'scope to point at the star HIP11932 (nondescript but in the right direction) so the camera, which was piggybacked on top of the 'scope, was also pointing in that general direction. It lies roughly on the border of the constellations of Cassiopeia and Perseus. Very difficult to pull the emission nebula out of the murk so the image is rather over-processed but here's what I found.

For comparison here's a thumbnail of one of the 2 minute exposures as delivered from the camera with no extra processing apart from a crop (the two crops are not the same):

The open clusters are easy enough to spot but there's the barest hint of the nebulae.

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There's the benefit of a narrowband filter for you. The signal to noise ratio is so much better even with an exposure time one third of what I needed and you're using a smaller aperture and slower f-number. Oh, and your camera doesn't block most of the Hα wavelength either! Win, win.

Bob.

P.S. When you've got a good selection of lights, darks, bias and flat frames to play with take advantage of PixInsight's free trial. You'll need to make liberal use of the tutorials (both video and text) unless you're a quicker study than I but the software is worth every penny. Still need PhotoShop, though.

Yes... but once you get that dedicated sensor I think the advantage will be very much be in your court again. I hate to think that 75% of the SLR sensor area is essentially doing nothing useful with narrowband. Or worse, it's just making it heat up faster. Also the 12MP sensor is now 3MP...

I am debating if a f/1.2 lens (more likely the 50mm) would be useful? But the clip filters I'm using aren't rated for operation beyond f/2 so I'm not sure what happens there. But would a 50mm f/1.2 or f/1.4 give better vignetting characteristics than the 50/2 Zeiss? I'll have to look into that.

Looking through the filter, as the angle is turned the colour shifts. So at some point I guess the response will drift far enough off to render it non functional in passing H-a.

I wonder, there must be software that can more intelligently process such raws? At least better than normal photographic software does.

So much to learn still!

Oh, I did download PixInsight... totally lost as to what any of it does! Definitely need to find those tutorials before the trial expires.

Actually I'm sort of serious. I've finally got my new astronomical camera (FLI ML16803) and filter wheel mounted up and while I'm still waiting for the new telescope there's no reason not to press ahead with the "Big Sky" side of the equation so I took a "first light" image during the day to check that I could actually reach infinity focus with my Pentax MF lens and that I understood enough about the hardware/software to actually capture an image! This is a 100% (664x442 pixel) crop from near the bottom left of a 4096x4096 pixel image taken with the CCD cooled to -25°C and an exposure time of 7/100ths of a second at f/2.8 through my Pentax 165mm 6x7 medium format lens. A 3nm Hα filter was in train to cut down the light intensity and even so I only just made it without having to stop the lens down!

So now I know the kit works it remains to get on with the serious business of using it at night. Hopefully with stars to work with I can improve the focusing slightly, using software tools designed for the job, especially as the shot above was taken through the conservatory window!!!
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But not tonight or for a couple after that. Cloudy.

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It occurs to me that in the previous post how the camera, filter wheel and lens are arranged isn't intuitive to anyone who isn't "into" astrophotography so here's a very preliminary look. I want to do some proper documentary shots later on but this one was taken today for more immediate use:

The lens is attached to the filter wheel which is bolted to a cradle I designed which in turn bolts onto a Losmandy plate which is compatible with the ASA telescope mounting. On the back of the filter wheel is the camera. Normally there would be several power cables and USB cables but I'll only be putting those in place just before use.

Bob.

P.S. The armillary, just visible at the bottom right, sits on top of the cast stone pier when the stainless steel plates which interface the mounting to the pier are removed.

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Darn it, or words to that effect. With stars in view, which are slightly further away than the TV aerial I showed a couple of posts back, this is what I got from a central 100% crop from a 10 second exposure at f/2.8 of Deneb with the Pentax lens hard up against the infinity stop:

The left hand image is at f/2.8 and the right hand one is a 30 second shot at f/5.6 with a less than attractive diffraction pattern around Deneb caused by the iris. These are just single exposures with no dark, bias or flat frames applied.

Close, but no banana, and backing the focus away from the infinity stop even by the tiniest amount just made it worse. The shot at f/5.6 showed how close the adaptor was to being perfect but close isn't good enough, unfortunately. I'm going to have to get Precise Parts to honour their guarantee and fabricate me a new adaptor. It will get done but by the time it does and then having to wait for the Moon to wane again I think that's my last chance at the North American nebula this year.

But even if the focus hadn't been out I would still have been stuffed as the clouds rolled in before I could have got properly going with my planned session for the evening.

Bob.

Update: Wouldn't you know it, the sky is now crystal clear. Aaargh! But Ashley at Precise Parts has already got back to me and the adaptor between the filter wheel and the Pentax lens goes back to Florida later this Saturday morning for reworking.

... Great read this thread, interesting technical stuff on every page! ^^

Thanks. It's always nice to know I'm not nattering away to myself and that throwing in titbits like the stars being further away than a local TV aerial are appreciated.

I was in two minds whether to post the continuing saga in this thread but it does highlight the fact that getting into "Big Sky Astrophotography" with anything other than a standard DSLR is not always a trivial task! The Precise Parts adaptor is already winging its way back across the pond and I'm hoping to have it back in two or three weeks time.

Went through this whole thread, but could you tell us what mounts you've used and what you're using for tracking? I'm interested in getting a tracking tripod for wide field astrophotography. I'm only somewhat bored with simple tripods. I noticed your signature says a tripod but doesn't look anything like the one you have in the post from years ago